Confined spaces such as building interiors and packaging may have limited exchange of moisture vapor with adjacent or external environmental spaces. Materials having moisture permeance that is variable when exposed to different ambient conditions can provide benefits in such fields as construction or packaging. Buildings are commonly equipped with thermal insulation in order to provide comfortable living conditions inside the building. In addition to insulation, buildings are also constructed with various air barrier materials that are installed between the sheathing and the exterior cladding to limit air and water infiltration into the structure of the building. These air barriers minimize heat loss through draft flow.
Building liner material that has a water vapor diffusion resistance of from 2 to 5 meters diffusion-equivalent air space width at a relative humidity of an atmosphere surrounding the vapor barrier between 30% and 50% has been disclosed. See, e.g., U.S. Pat. No. 6,808,772, U.S. Pat. No. 6,878,455, U.S. Pat. No. 6,890,666, US 2003/0215609, and WO2002/070351.
In addition to air barriers, vapor barriers are also used for controlling flow of moisture. For example, the moisture flow is generally from inside to outside when the exterior air is cold and dry (winter) and from outside to inside when the air is hot and humid (summer). In regions where these conditions prevail, it is common practice to use a vapor barrier such as polyethylene (PE) film on the warm side of the insulation, generally behind the gypsum wallboard, to prevent moisture diffusion from the interior heated space to the exterior cold environment. Such vapor barriers reduce the potential for moisture condensation in the interior wall cavity space as the water vapor is exposed to the cold exterior temperatures. Moisture condensation causes wood decay, promotes mold growth, reduces insulation value, and consequently must be eliminated whenever practical.
Although the use of PE film as a vapor barrier reduces moisture condensation during exterior conditions of cold temperatures and low humidity, the same vapor barrier increases the likelihood of moisture condensation at the insulation-PE film interface during hot and humid exterior conditions where moisture flow is from outside to inside. Therefore, it is desirable to have a material that acts as a vapor barrier during winter but facilitates vapor permeation during hot, humid summer conditions. It is also desirable to have a vapor barrier that can become vapor permeable under humid conditions in the wall cavity caused by unexpected uncontrolled events like water leaks into the wall cavity, to facilitate drying of the wall cavity by inward moisture flow as well as outward flow.